My understanding of it is that its up to the rom developer to include file type support. As an example, CM7 supports FLAC and every music player I've tried supports it but if I switch to a rom without flac support I can't play it with any.

I really like the wp7 home screen, which I use launcher7 to emulate, so I thought I'd give this a try. Not a fan of the UI, and the player was pretty meh. Couldn't even get the order right in an album. Might be interesting to see new version, but I guess you either love the UI, or hate it. I don't love it.